Well, I'm sure that it isn't an extended selection created by dragging. I have some additional infos now...
After further investigations, it seems to be a focus issue, because clicking on the title bar resolves the selection problem, well until it happens again. I have still no idea what really triggers it.

When I click on the title bar, the selection remains the same, but if I click on another row, then the row gets selected and the existing wrong selection disappear. If I click on another row without clicking on the title bar in such cases, the selection continues to get wrongly extended as reported.

I'm currently using latest svn: base, gui, back (cairo) and gorm -r26829

When I click on a browser cell/row to get it selected (no key modifiers involved) , I often get all the rows/cells above selected in the browser column. When the selection is wrongly extended like this, I also get no display of the content of the selected item (a branch cell in this case). Hence it doesn't look like a display update problem.
Although I get this bug many times a day, I haven't been able to find a way to reproduce it reliably.
Once the selection is in a wrong state, it's usually difficult to prevent the bug to repeat on the next clicks either in the same browser or others (if several browsers are visible on screen together as it can be the case with Gorm for example). Restarting the running application seems to be the only reliable workaround.
You can find a Gorm screenshot of the issue in attachment, the bug isn't Gorm-specific though. In the screenshot, the click was on the last selected and focused cell.

This bug has been around for a month or more, I got it with previous revisions too. iirc I also get it with art backend. I didn't report it earlier because I hoped to figure out a reliable way to reproduce it :-/